🥧Pie Charts

A cake must be shared out to the whole group — who gets how much? The clearest place to draw the answer is on a circle: the whole circle is the cake, and each person gets a wedge. A pie chart draws exactly this, "the whole and its shares": who gets the big slice is visible at a glance.

One circle holds everything

A pie chart draws the whole as a complete circle, then slices it into wedges, one per part.

There is only one rule: all the wedges must piece the circle back together. In other words, the percentages of all parts must add up to exactly $100\%$.

• A wedge worth $25\%$ is exactly a quarter of the circle;
• All the other wedges together must make the remaining $75\%$ — no more, no less.

Reading wedges: bigger angle, bigger share

A wedge's size is decided by its angle at the center. The full circle is $360°$, so:

• a wedge with a $90°$ angle holds $90 \div 360 = 25\%$ of the whole;
• to claim half the pie ($50\%$), a wedge needs $180°$ — a perfect half circle.

The same wedge can be described three ways:

• as a fraction: $\frac{1}{4}$ of the whole;
• as a percentage: $25\%$;
• as an angle: $90°$.

All three say the same thing: $\frac{1}{4} = 25\%$ and $25\% \times 360° = 90°$. Converting between fractions and percentages is old news from Fractions and Percentages — a pie chart simply moves it onto a circle.

Worked example: from headcounts to one wedge

Pie or bar? Pick the right tool

• The pie's strength: showing each part's share of the whole — how the 24 hours of a day are spent, where the family money goes.
• The bar's strength: comparing categories and seeing by how much they differ — see Bar Graphs.
• The pie's weakness: with many categories (more than five or six) the wedges blur together, and when two shares are close — say $52\%$ versus $48\%$ — the eye can barely rank them.

📊 Bar🥧 Pie

🍎🍎🍌🍇🐶🐱🐦🐟⚽🏀🏐

Switch to pie mode and watch the same data change costume: who owns the big share pops out, but exact gaps suddenly need squinting. The lesson: decide what question you want to answer, then choose the chart.

Check yourself

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Quick quiz

↺ Try again

1. 1. In any pie chart, the percentages of all wedges must add up to…

   100%50%360%

2. 2. A wedge has a 90° angle at the center. What percent of the whole is it?

   90%25%10%

3. 3. In a class of 40, ten students pick blue. What share does blue get on the pie?

   25%40%10%